May I fire someone based on at-will status?

Yes, but relying solely upon at-will employment to support a termination decision does not offer protection against most types of wrongful termination lawsuits or charges of discrimination, harassment or retaliation filed with state and federal agencies.

California is an At-Will State

All employment in California is employment at-will, unless the employer and employee have agreed otherwise. California's Labor Code provides that "an employment, having no specified term, may be terminated at the will of either party on notice to the other."